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I installed new light fixture in a mudroom and a Lutron TGCL-153PH-WH dimmer toggle switch for it. I'm adding the switch into a gangbox where there is an existing 3-way light switch for the hallway.

My wiring: took the supply wire off the old switch (yellow wire), added a pigtail, connected one tail back to the old switch, and connected the other tail to the new switch. Wired the neutral from the new light to the other neutrals in the gangbox.

In this manner, however, the new switch only operates when the old switch is 'on'. But I want these switches to operate independently. I'm not sure where I went wrong. Any ideas? enter image description here

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  • Can you get us reliable colors on those traveler wires? Dec 19, 2021 at 21:02
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    Where are you located? Most residential wiring is cables, not conduit, unless required. Your old wiring is conduit. If conduit is required then if an inspector ever finds the cable you added it will have to be removed and it will be a clear indication of work done improperly which may raise other questions. Dec 19, 2021 at 22:08

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A set of 3-way switches is, essentially, power -> switch 1 -> switch 2 -> lights.

Each 3-way switch has one of two possible combinations:

  • Incoming hot (always on) + 2 travelers
  • Switched hot (controlled by the switches) + 2 travelers

If you are in that first configuration, you can pull hot from the switch and pigtail it to the new switch and everything will work great.

If you are in that second configuration, and every 3-way switch group will have one of those somewhere, then that hot you pulled from the switch is actually switched hot and only live when the 3-way switches are in a combined "on" status.

There are two possible solutions:

  • Since your wiring appears to be in conduit, you can fish an always hot wire between the two 3-way switches, connect it to always hot (pigtail like you tried here) at the other switch and to the new switch. And then disconnect the pigtail here (just wire straight to the 3-way switch as before).
  • Replace the 3-way switches with smart switches that only need one traveler. Then you can use the 2nd traveler as an always hot.

You should also make 100% certain you are allowed to use NM cable instead of conduit. It is unusual, but not impossible, to have conduit where it is not required. But very often it is there because it is required. Assuming that NM cables are allowed, you need to connect the ground wire to the metal box with an a grounding screw.

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the most obvious explanation is that you didn't pick the right wire as the supply, but instead picked the switched hot or one of the travelers. (depending oh how I interpret your description of the problem)

assumiong you correctly identified the common terminal of the 3way switch the supply is most-likely the black wire that passes through the box.

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